The cystatin superfamily comprises a group of cysteine proteinase inhibitors which are widely distributed in human tissues and body fluids, and which form tight and reversible complexes with cysteine proteinases such as cathepsins B, H, L, and S. The cystatins are most likely involved in the regulation of normal or pathological processes in which these proteinases participate. Thus, cystatins may influence the intra- and extracellular catabolism of proteins and peptides (Barret, A. J. and Kirchke, H., Methods Enzymol., 80:535-561 (1981)), regulate proteolytic processing of pro-hormones (Orlowski, M., Mol. Cell. Biochem., 52:49-74 (1983)) and pro-enzymes (Taugner, R., et al., Histochemistry, 83:103-108 (1985)), protect against penetration of normal tissues by malignant cells (Sloane, B. F., Semin. Cancer Biol., 1:137-152 (1990)) or microorganisms (Bjorck, L., et al., Nature, 337:385-386 (1989) and Bjorck, L., et al., J. Virol., 64:941-943 (1990)) and modulate local inflammatory processes in rheumatoid arthritis (Mort, J. S., et al., Arthritis Rheum., 27:509-515 (1984)) and purulent bronchiectasis (Buttle, D. J., et al., Scand. J. Clin. Lab. Invest., 50:509-516 (1990)).
The cystatin superfamily has been sub-divided into families I, II and III (also called the stefin, cystatin and kininogen families, respectively), each with members differing from those of the other families in structural organization and biological distribution (Barret, A. J., et al., Biochem. J., 236:312 (1986)). The family I cystatins A and B are small proteins consisting of single polypeptide chains of about 100 amino acid residues without disulfide bridges. The family II cystatins consist of polypeptide chains of approximately 120 amino acid residues with two intra-chain disulfide bonds. Finally, the family III cystatins, the kininogens, display a higher degree of structural complexity characterized by the presence of three family II cystatin-like domains, each with two disulfide bridges at positions homologous to those in family II cystatins (Muller-Esterl, W., et al., Transbiochem. Sci., 11:336-339 (1986)). Family I and II cystatins are mainly present intracellularly and in secretory fluids (Abrahamson, M., et al., J. Biol. Chem., 261:11282-11289 (1986)), whereas kininogens are highly concentrated in blood plasma (Adam, A., et al., Clin. Chem., 31:423-426 (1985)).
At least one type II cystatin, designated cystatin C, appears to be expressed in all tissues (Abrahamson, M., et al., Biochem. J., 268:287-294 (1990)). In contrast, S-type cystatins are found predominantly in saliva (Abrahamson, M., et al., J. Biol. Chem., 261:11282-11289 (1986)). Cystatins and derivative peptides possess antibacterial and antiviral activities (Bjorck, et al. (1989, 1990)), consistent with their presence in secretions bathing epithelial surfaces directly exposed to the environment. The cystatins may also modulate the immune response. This could occur directly, by inhibiting cysteine protease releases by macrophages (Bieth, J., Cysteine Proteinases and Their Inhibitors, V. Turk, ed. (Walter De Gruyter & Company, New York) pp. 693-703 (1986)), or indirectly, by inhibiting the chemotaxic response and the phagocytosis-associated respiratory burst of the cells (Leung-Tack, et al., Biol. Chem., 371:255-258 (1990)). This data suggests that type II cystatins might perform a variety of protective functions at epithelial surfaces. The human type II cystatin gene family consists of at least seven members.
The disease hereditary cystatin C amyloid angiopathy (HCCAA) is associated with a Glu to Leu mutation in the gene encoding cystatin C. This leads to deposition of amyloid fibrils comprised of this mutant cystatin C in the cerebral arteries, which appears to cause fatal hemorrhaging (Ghiso, J., et al., PNAS, USA, 83:2974-2978 (1986)).
The polypeptide of the present invention has been putatively identified as a CysE as a result of amino acid sequence homology to cystatin C and on conservation of cystatin-like functional motifs in its amino acid sequence.